The chairman of the House Permanent Select Committee on Intelligence said Monday that he doesn't believe Trump Tower was wiretapped, but left open the idea that there was other surveillance against President Trump by the Obama administration.

"Let me be clear, I've been saying this for several weeks, we know there was not a physical wiretap of Trump Tower. However, it's still possible that other surveillance activities were used against President Trump and his associates," Chairman Devin Nunes, R-Calif., said during Monday's hearing on Russia's meddling in the U.S. election.

Nunes later told reporters after the hearing that until his committee reviews all the intelligence products related to the alleged wiretapping, he is not ruling anything out.

"Until we get all the names that were unmasked and we look at all the intelligence products that had American names in it, we won't know until we get to see that," Nunes said.

Trump has said he was surveilled somehow, but has left it to Congress to discover how, and has declined to offer Congress evidence for his claim. Trump has instead cited press reports that discussed the idea of surveillance against him.

More broadly, Nunes said the failure of the U.S. intelligence community to get a handle on Russia has marked a large failure for the intelligence community and committee.

"The fact that Russia hacked U.S. election-related databases comes as no shock to this committee," said Nunes. "We've been closely monitoring Russia's aggression for years. A year ago, I publicly stated that our inability to predict Putin's regime plans and intentions has been the biggest intelligence failure that we've seen since 9/11, and that remains my view today."

Democrats, meanwhile, focused on whether Trump was colluding with Russia somehow to help defeat Hillary Clinton in the election. Nunes' Democrat counterpart, Adam Schiff, laid out a timeline that he believes shows strong circumstantial evidence that there may have been coordination, and highlighted the string of email leaks from the email account of John Podesta by WikiLeaks, and other leaks from a hacker named Guccifer 2.

Schiff also highlighted former national security adviser Mike Flynn's discussion with the Russian ambassador, which ultimately led to Flynn's resignation as national security adviser, and revelations that then-Sen. Jeff Sessions met twice with Russian Ambassador Sergei Kislyak.

"Last summer, at the height of a bitterly contested and hugely consequential presidential campaign, a foreign, adversarial power intervened in an effort to weaken our democracy, and to influence the outcome for one candidate and against the other," Schiff said. "That foreign adversary was, of course, Russia, and it acted through its intelligence agencies and upon the direct instructions of its autocratic ruler, Vladimir Putin, in order to help Donald J. Trump become the 45th President of the United States."